Field of the Invention
The present invention relates to a method for producing disposable, thin-walled molded articles, such as cups, plates, fast-food packages, trays, flat sheets and the like by applying a starch-based baking composition onto the lower mold part of a multi-part, preferably two-part mold, baking and conditioning to a moisture content of 6% by weight to 22% by weight, the baking composition, in addition to water and a starch or starch mixtures and/or flour or flour mixtures and/or starch derivatives, contains a release agent, namely one or more medium- or long-chained, optionally substituted fatty acids and/or salts thereof and/or acid derivatives thereof, such as acid amides, and/or a polymethyl hydrogen siloxanes, and optionally:
thickening agents, such as swelling starch, pregelatized starch or baking waste, and/or guar flour, pectin, carob seed flour, carboxymethylcellulose and/or gum arabic; PA0 fibrous materials, such as cellulose-rich raw materials, plant materials, fibers of plastic, glass, metal and carbon materials; PA0 nonfibrous filler materials, such as calcium carbonate, coal, talcum, titanium dioxide, silica gel, aluminum oxide, shellac, soy protein, powdered wheat gluten, powdered egg white from chicken eggs, powdered casein; PA0 powdered pigments; PA0 a zirconium salt, preferably ammonium zirconium carbonate and/or ammonium zirconium acetate, as structural stabilizers; PA0 preservatives and antioxidants. PA0 1. Over the course of long-term storage and in heated rooms during the winter, the relative humidity is often below 20%, or even below 10%. PA0 2. In molded articles or molded article parts that are exposed to increasing bending strain: as an example, drinking glasses (compression strain during use) or two-piece hinged molded articles ("clamshells"), where the hinge is likely subjected to repeated opening and closing operations (requiring increased flexibility). PA0 1. After baking, the products do not adhere to the baking molds, even though polyvinyl alcohol is a known hot-melt adhesive and softens above its glass transition temperature of about 80.degree. C. This might be ascribed to the fact that polyvinyl alcohol, on heating and drying at high temperatures (below the melting point, which depending on the type, is between 185 and 230.degree. C.), crystallizes rapidly. PA0 2. The "baked" starch and polyvinyl alcohol foams remain partly phase-separate. Electron micrographs of the surface of such molded articles show swollen starch grains embedded in a polyvinyl alcohol matrix, while the interior instead looks homogeneous. The mixture remains phase-separate, since in contrast to extrusion no mixing action or only slight mixing action ensues during baking, and polyvinyl alcohol and starch are largely incompatible. Polyvinyl alcohol, which is a stronger and more-flexible polymer than starch, is suspected of binding together the swollen starch grains and thus increases the mechanical strength and stability of the molded articles. Extruded starch and polyvinyl alcohol foams, conversely, undergo intensive mixing, which is associated with the dissolution of the starch grain structure. PA0 3. Since the final mold is formed directly during the baking process, cross-linking aids which increase the stability and water resistance can be admixed. This is not possible in the extrusion process, because a highly cross-linked material would not be adequately flowable.
Molded articles produced with these prior art baking compositions still have a number of disadvantages. For instance, at relatively low humidity, approximately below 50%, in conjunction with slow moisture desorption, these molded articles exhibit ever-increasing brittleness.
This makes itself felt especially disadvantageously in two areas:
Another disadvantage of molded articles of starch, precisely in comparison with cellulose-based materials (paper, cardboard) is the virtually complete loss of tear strength if they become soaked.
Polyvinyl alcohol is a biodegradable synthetic polymer that has long been used for water-soluble films, in paper processing, and in textile impregnation. Its use together with types of starch is known from the production of cast films and from extrusion technology.
U.S. Pat. No. 3,312,641 to Young teaches that films cast from aqueous solution and comprising amylose or amylose-rich starch and polyvinyl alcohol, have greater tensile strength and are more stretchable, at 23 and 50% relative humidity, than pure starch films.
U.S. Pat. No. 3,949,145 to Otey describes similar improvements in sheets made of normal cornstarch (27% amylose), used jointly with formaldehyde for cross-linking.
U.S. Pat. No. 5,095,054 to Lay et al. and European Patent Application EP 0 400 531 A1 (Bastioli et al.) describe the melt extrusion of starch, water and polyvinyl alcohol to form a homogeneous melt. These references state that improved dimensional stability at high humidity is found.
According to U.S. Pat. No. 4,863,655 to Lacourse, a homogeneous melt of amylose-rich starches, water and up to 10% polyvinyl alcohol is again extruded. The result obtained is an expanded foam (filler chips).
Methods for producing foamed molded articles of starch from baking compositions by gelatinization without creating homogeneous melts beforehand are known from U.S. Pat. No. 5,376,320 to Tiefenbacher et al. (corresp. European Patent Disclosure EP 513 106 B1).
A decrease in brittleness at relatively low humidity and an increase in flexibility and water resistance of such molded articles with starch is desirable and could greatly expand their fields of application.
However, the art has had reservations with regard to the well-known adhesive action of polyvinyl alcohol.
In a baking method at temperatures of around 200.degree. C., the question of thermal stability and formation of residues on the hot mold surfaces must also be taken into account.
Not least because of the known rheological properties of the starch--"dilatory" viscous behavior with the danger of seizing of pumps from friction at high viscosity, for instance--the use of high-viscosity additives, such as polyvinyl alcohol, appears inadvisable. On the other hand, with major dilution with water in this process technique and an attendant decrease in the proportion of dry substance and increase in the water "leavening" in the baking compositions, it is known that only lightweight, fragile molded parts, and in some cases only molded parts that are not cohesive, or parts that foam markedly out of the mold can now be produced.
Surprisingly, it has now been discovered that most of these prejudices are unjustified, as long as certain factors, described in further detail below, are taken into account.